The present invention is directed to bicycle chainwheels and, more particularly, to a bicycle chainwheel of the type having missing teeth or teeth shaped differently from other teeth on the chainwheel.
FIG. 7 is a view from the bottom of part of a known rear multi-gear chainwheel mounted on the rear hub of a bicycle. The drive face of two of the ordinary teeth 4 is removed to produce smooth-disengagement teeth 5 in which the tooth width d thereof is smaller than the tooth width D of the ordinary teeth 4.
In general, the pitch between the ordinary teeth 4 is set to p-.alpha. (where p is the pitch of the chain 3 hooked onto the chainwheel 1'), which is somewhat smaller than the chain pitch p so that the engagement with the chain 3 will be smoother. Furthermore, the sum of the pitches P between the ordinary tooth 4 immediately ahead of the smooth-disengagement teeth 5 and the ordinary tooth 4 immediately behind is set to be the sum of the pitches p-.alpha. between the ordinary teeth 4, i.e., (p-.alpha.).times.3.
Similarly, although not depicted in the figures, there are also chainwheels in which some of the numerous teeth of the chainwheel were removed to facilitate shifting, or in which some of the teeth are bent into chain guard stays. In these chainwheels the sum of the pitches P between the ordinary teeth immediately ahead of and behind the removed or bent teeth is also set to be (p-.alpha.).times.(N+1). In this case N is the number of continuous removed teeth or the number of continuous bent teeth.
In a chainwheel composed solely of ordinary teeth 4 (i.e., no smooth-disengagement teeth 5 are provided and none of the teeth are bent), the drive force is transmitted when the roller 3a of the chain 3 comes into contact with the drive face of the forwardmost ordinary tooth 4 in the drive direction. When the chain 3 is disengaged from this forwardmost ordinary tooth 4, the next roller 3a of the chain 3 comes into contact with the next ordinary tooth 4, but there is a gap of .alpha., which is the difference between the chain pitch p and the tooth pitch p-.alpha., between the drive face of the next ordinary tooth 4 and the next roller 3a. Consequently, the chain 3 or the chainwheel moves forward relatively by this distance .alpha., after which the drive face of the next ordinary tooth 4 and the next roller 3a come into contact. This occurs with each tooth, so the presence of the gap .alpha. generates a sound referred to as clicking.
In the above chainwheel 1' having the smooth-disengagement teeth 5, the smooth-disengagement teeth 5 produced by removal of the drive face do not participate in the transmission of drive force as much as do the ordinary teeth 4, and those teeth that are completely removed or bent do not play any part in the transmission of drive force. Nevertheless, the chainwheels are still constructed with the gap of .alpha. for each tooth as noted above. Thus, when an .alpha. equal to the number of smooth-disengagement teeth 5 is added and there are two continuous smooth-disengagement teeth 5 (for example), then a gap of 3.alpha. will be present between the roller 3a and the ordinary tooth 4 immediately behind the smooth-disengagement teeth 5. The sudden motion of the chain or chainwheel as it moves across this relatively large gap, in contrast to a gap of .alpha. in the normal case, interferes with smooth pedaling and causes a very unpleasant noise. The same holds true for chainwheels having completely removed or bent teeth.